The present invention relates to an electrophotographic image forming apparatus for forming a color image in an intermediate transfer system, and a transferring method applied to that image forming apparatus.
One method for forming a color image in electrophotographic image forming apparatuses is an intermediate transfer system. The intermediate transfer system is a system in which a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are transferred to an intermediate transfer member (first transfer), and then these toner images attached to the surface of the intermediate transfer member are transferred all at once to a recording sheet (second transfer).
However, in an intermediate transfer system it is difficult to set the electric field strength during the second transfer. This is because an appropriate range of the electric field strength in the second transfer varies in accordance with the charge amount of toner particles attached to the intermediate transfer member per unit area. In addition, the charge amount of toner particles per unit area is affected by the thickness of a toner image formed on the intermediate transfer member, and thus the appropriate range of the electric field strength in the second transfer is sometimes different from position to position even within the same toner image.
When performing the second transfer outside the appropriate range of the transfer electric field strength, toner particles to be transferred to a recording sheet in the second transfer are more likely to remain on the side of the intermediate transfer member. When toner remains on the intermediate transfer member, a desired density cannot be attained in a monochrome image. Further, in the case of a color image, the ratio at which color toner images are mixed changes, and thus the color balance of a reproduced full-color toner image is degraded.
In order to address this problem, JP H08-292661A has disclosed a configuration in which a photoreceptor drum is used as a second transfer device, so as to satisfactorily perform second transfer when forming a color image in the intermediate transfer system. In this configuration, a latent image potential based on the image data is formed on the circumferential face of the photoreceptor drum as the second transfer device when toner images are transferred all at once from the intermediate transfer member to paper. It is described that the configuration enables a plurality of kinds of electric fields in the second transfer each having an appropriate strength to be set for the portions of the toner image, and thus it is possible to satisfactorily perform the second transfer regardless of the non-uniformity in the charge amount of toner particles attached to the intermediate transfer member per unit area.
However, the invention relating to JP H08-292661A cannot be applied when the photoreceptor drum is not used as the second transfer device, and thus the types of image forming apparatuses to which the invention can be applied are limited.
It is an object of the present invention to provide an image forming apparatus and a transferring method with which the second transfer strength in the color image forming process in an intermediate transfer system can be set easily with a simple configuration.